(a) Field of the Invention
The present invention relates to a photo-reactive norbornene-based copolymer and a method of preparing the same. More specifically, the present invention relates to a photo-reactive norbornene-based copolymer which has superior miscibility to various organic solvents or additives while exhibiting superior liquid crystal alignment property and can be preferably used to an alignment layer of liquid crystal display device, a method of preparing the same, and an alignment layer including the same.
(b) Description of the Related Art
Recently, as the liquid crystal display (LCD) is enlarged and the use is gradually extended from a personal use such as mobile phone or lap-top computer to a family use such as wall-mountable TV and the like, high-definition, high-fidelity, and wide viewing angle are being required to the LCD. Particularly, thin film transistor-liquid crystal display (TFT-LCD) which is operated by a thin film transistor can realize moving images of high-definition because it operates each pixel independently and shows very good response time of liquid crystal, and thus the application range thereof is gradually extended.
In order to use the liquid crystal as an optical switch in the TFT-LCD, the liquid crystal on the innermost thin film transistor layer of the display cell must be pre-tilted in a certain direction, and the alignment layer is used for this.
For the liquid crystal alignment, a rubbing process of coating a heat-resistant polymer such as polyimide and the like on a transparent glass so as to form a polymer alignment layer, and rubbing the alignment layer with a rotating roller which is rolled up in a rubbing cloth such as nylon or rayon by rotating the roller with high speed so as to align the layer has been applied before.
However, the rubbing process may give mechanical scratches on the liquid crystal alignment layer or generate high static, and thus it may destroy the TFT. Furthermore, the fine fibers from the rubbing cloth generate defects and it becomes an obstacle to the improvement of production yield.
A newly invented liquid crystal alignment method for overcoming the problem of the rubbing process and achieving an innovation in the side of production is a liquid crystal alignment by light such as ultra-violet (UV) (hereinafter ‘photo-alignment’).
The photo-alignment means the mechanism of forming a photo-polymerizable liquid crystal alignment layer that photo-sensitive groups bonded to a certain photo-reactive polymer are photo-reacted by a linear polarized UV, the main chain of the polymer is aligned in a certain direction during the process, and the liquid crystal is aligned finally.
Representative examples of the photo-alignment are the photo-alignment process according to photo-polymerization published by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31., 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The photo-alignment polymer used in the patent and articles are mostly a polycinnamate-based polymer such as poly(vinyl cinnamate) (PVCN) or poly(vinyl methoxycinnamate) (PVMC). In the case of photo-aligning the polymer, the double bonds of the cinnamate form a cyclobutane through a [2+2] cycloaddition reaction by a UV radiation, anisotropy appears and the liquid crystal molecules are arranged in one direction according to this, and the alignment of liquid crystal is induced.
In addition to, Japan Patent Publication No. Hei 11-181127 discloses a polymer having a side chain including photo-sensitive group such as cinnamic acid group in the main chain of acrylate or methacrylate, and an alignment layer including the same. Furthermore, Korea Patent Publication No. 2002-0006819 discloses the use of an alignment layer made of polymethacrylic polymer.
However, the prior photo-alignment polymers disclosed above had a disadvantage of that the stability of the alignment layer was deteriorated due to low thermal stability of the polymer main chain, or the liquid crystal alignment was insufficient because the photo-reactive functional group was not sufficient. Furthermore, the alignment layer is generally formed from a composition for alignment including a binder resin and an organic solvent in addition to the photo-alignment polymer, the photo-alignment polymers known before were poor in the solubility to the organic solvent or the miscibility to the binder resin, and there were many cases of that the coating property of the composition for alignment decreased, or various coating methods or additives are hard to be applied to the composition.